Here are a few possibilities for your symptom of loose bowel movements after eating.
You may have irritable bowel syndrome, which often causes normal bowel reflexes to be overactive and is often associated with loose stools. Secondly, venlafaxine may contribute to loose stools/diarrhea. Though this may not be the only cause, it may be making your problem worse.
Another possibility is that you have a malabsorption syndrome. Due to various problems such as deficient digestive enzymes or inflammation of the intesitines (such as is seen in a disorder known as Celiac Sprue), many people develop diarrhea.
In order to determine the cause for your symptoms, your doctor may wish to check for other tests such as pancreatic function tests, testing for Sprue, and possibly do imaging of your GI tract with endoscopy. You may wish to discuss your symptoms with an internal medicine doctor and may need a referral to a gastroenterologist.
While you should try to find out the cause of your symptoms, sometimes medications such as immodium and supplements such as metamucil which add "bulk" to your bowel movements may help with diarrhea.
I hope this has helped.